The invention relates generally to devices for charging, formatting, and reconditioning batteries. More particularly, the invention relates to methods and devices for rapidly charging and/or reconditioning a battery that provide particular staging sequences that primarily consist of charge pulses, discharge pulses, such as by applying a load across the battery, and wait or rest periods. Batteries are commonly used to provide a direct-current (dc) source of electrical energy in a wide variety of applications. A battery generally consists of a plurality of cells grouped together in a common container and electrically connected to provide a particular dc source. For example, four 1.5 volt cells rated at 1 ampere (amp) may be series connected to provide a 6 volt dc source rated at 1 amp. Cells may also be connected in parallel, e.g., four 1.5 volt cells rated at 1 amp connected in parallel provide a 1.5 volt dc source rated at 4 amps, or in a combined series-parallel fashion. The cell consists of two electrodes, one connected to a positive terminal and the other connected to a negative terminal, which serve as conductors through which current enters and leaves the battery. The electrodes or plates are surrounded by an electrolyte that acts upon the electrodes in a manner dependent upon the nature of the materials used to comprise the battery.
A battery may be a primary or a secondary cell type. The present invention is essentially concerned only with secondary cell type batteries, which may be recharged by forcing an electrical current through the battery in a direction opposite to that of discharge. Batteries store current, the storage capacity of a battery is generally rated according to ampere-hour capacity, e.g. a battery that delivers an average of 10 amps without interruption over a 2 hour period, at the end of which time the battery is completely discharged upon reaching a low voltage limit, has a capacity rating of 20 (10.times.2) amp-hours. If discharged at a faster rate, e.g. over one hour instead of two, the battery will deliver less than the rated capacity.
Battery charging is, in its simplest terms, accomplished through delivery of a current to a battery, thereby ionizing the plates to opposing potentials (voltages or electrical pressures). With a linear charger, this is achieved through the use of a marginally higher charger voltage vs. what the battery's maximum rest voltage is. Usually, in the most rudimentary charger types, minimal consideration is given to regulation, whether it be current or voltage. The battery is simply allowed to drift up to its maximum potential. This is useable, albeit slow, and also does not treat the battery equally over time. When the battery is lowest, the most amount of current flows. If you were to divide the time that it takes for the battery to charge by five, the first time segment would appear to have charged the battery by about 70%, the next segment would be up to 90%. The remaining three time segments gradually approach the 100%.
When charging a battery, it is preferred to use a charge voltage that is only marginally higher than the full battery potential. This is because any voltage above that will be leaked across the electrolyte of the battery and not stored. Energy not stored, having nowhere else to go, is converted to heat. Roughly, this would be (E.sub.c -E.sub.R).times.I.sub.c =P.sub.w where E.sub.c is the charging voltage, E.sub.R is the battery resting voltage, I.sub.c is the charging current, in amperes, and P.sub.w is the power wasted in watts. This P.sub.w is converted to heat since it cannot be absorbed by the battery. More advanced chargers use a pulsing technique to send bursts of energy to the battery at higher voltages. While this does get the job done quicker, the effects are to gradually destroy the battery by heating, and eventually evaporating, the electrolyte inside.
An alternative path is to use a constant current source, assuming that the battery is capable of accepting a set amount of current at any time. These chargers regulate current by automatically adjusting the voltage so a predetermined amount of current is delivered. Volume of current is the design factor, not the voltage of the battery. Unfortunately, the results here can be less than desirable. If the amount acceptable to the battery is overestimated, the battery becomes less willing to accept the current. The charger instantly compensates by raising the charge voltage. The battery heats up, which further exacerbates the situation by causing the battery to become even less able to accept further charging. The cycle continues. Without limits, this scenario could escalate to destruction of the battery.
During the charging process, ionization of the plates, negative ions to the positive plate and positive ions to the negative plate, occurs which impedes the further transfer of ions to the plates and therefore further charging of the battery. The battery develops increased resistance, in the form of impedance, to charging as the battery's charge increases. Recent battery charging techniques attempt to more rapidly charge a battery by applying discharge pulses intermittently between charging pulses to depolarize the plates of the battery. The discharge pulse intermittently removes ions from the plates, thereby permitting the flow of additional ions which will then transfer additional charge to the plates upon subsequent charge pulses.
During a given charge pulse, the voltage of the battery will be at its greatest at the end of the charge pulse. Battery chargers of the prior art apply the discharge pulse after the charge pulse to depolarize the battery while at its highest voltage level to prepare the battery for the subsequent charge pulse. One such prior art charger is disclosed in U.S. Pat. No. 4,829,225 (Podrazhansky) and calls for the discharge pulse to immediately follow the charge pulse. Wait or rest periods are applied intermediate the charge and discharge pulses to stabilize the battery during charging, this is especially important in charging applications for nickel-based batteries. The durations of the charge, discharge, and wait periods are generally in the order of tenths of seconds to a few seconds. The load across the battery generally takes the form of a depolarization pulse, the current level of which is of the same magnitude or greater than the current level of the charging pulse. Generally, the charging pulse is of substantially longer duration than that of the load or depolarization pulse. Generally, the duration of the rest period, or stabilization period, is greater than the duration associated with the load or depolarizing pulse.
The magnitude of the charge or load, the duration of the charge, load, and rest stages, and the particular sequencing of the three stages is dependent upon the particular battery type being charged or reconditioned Further, the sequencing, magnitude, and duration of these stages may be varied during the process of charging or reconditioning the battery. Generally, the batteries are measured while under load to determine the level of charge as this tends to give a more accurate measurement of the battery that would otherwise be colored by the battery's impedance and the charge delivered.
Conventional battery chargers merely deliver steady voltages and slowly ionize the plates in the battery. A problem with this is that the battery cannot absorb all of the energy delivered and the charging process takes an excessive amount of time to complete. Part of the reason is that the charger is forced to deliver a certain voltage level, just above the normal capacity, and the static resistance of the battery makes it so that a certain amount of current is delivered. Initially, the amount of such current is often too much for an ongoing charge. In addition, at various points during the charging process, the resistance of the battery changes, further complicating efficient battery charging. Whenever there is more power being delivered to the battery than it can absorb, the excess is spent by converting it to heat.
Some modern pulse chargers disclose the use of load, charge, and wait/rest periods during charging. Although such pulse chargers do "shock" a charge into a battery, the amount of positive-going rail-to-rail voltage measured across the battery is not very significant. If the bakery were discharged by a load just before it received a charging pulse, the positive-going rail-to-rail voltage would be increased, thereby "shocking" the battery even harder. Because pulse chargers use non-linear power delivery, static battery resistance cannot be a factor. However, battery impedance is a factor and is affected by the frequency and duty cycle associated with the pulse charges.
One problem associated with prior art battery chargers, such as so-called wall-wart type chargers, concerns the amount of time required to fully charge a battery. Such known chargers generally require a period of 12-24 hours of continuous charging to fully charge a typical battery. This results in undesirable, extended down-times for devices being served by the battery being charged or the expense of purchasing and maintaining multiple batteries.
Another problem associated with the prior art is that batteries charged by prior art chargers typically suffer the undesirable effect of steadily decreasing charge capacity, or memory, the so-called "memory effect." As the battery is charged over numerous occasions, its charge capacity gradually decreases. The memory effect relates to a permanent increase in the impedance of the battery resulting in increased resistance to charging. Eventually the battery cannot maintain any appreciable charge and must be discarded or reconditioned.